Three goals are envisioned. First, we wish to learn how triiodothyronine helps to induce nucleolar hypertrophy and nuclear DNA replication in the hepatocytes of the unoperated rat. Second, we hope to elucidate the relationship between sulfur amino acids and the regulation of RNA polymerase I activity and DNA formation. Finally, it is planned to continue our studies on a nuclear DNA polymerase that shows promise of being the DNA replicase of the parenchymal cell of liver.